Wireless communication systems are widely used to provide various types of communications. For example, voice and/or data are provided by the wireless communication systems. A conventional wireless communication system provides multiple users with one or more shared resources. For example, the wireless communication system can use various multiple access schemes such as code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA).
An orthogonal frequency division multiplexing (OFDM) scheme uses a plurality of orthogonal subcarriers. Further, the OFDM scheme uses an orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). A transmitter transmits data by performing IFFT. A receiver restores original data by performing FFT on a received signal. The transmitter uses IFFT to combine the plurality of sub-carriers, and the receiver uses FFT to split the plurality of subcarriers. According to the OFDM scheme, complexity of the receiver can be reduced in a frequency selective fading environment of a broadband channel, and spectral efficiency can be improved through selective scheduling in a frequency domain by utilizing channel characteristics which are different from one subcarrier to another. An orthogonal frequency division multiple access (OFDMA) scheme is an OFDM-based multiple access scheme. According to the OFDMA scheme, a radio resource can be more efficiently used by allocating different subcarriers to multiple users.
Recently, to maximize performance and communication capability of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is classified into single-user spatial multiplexing and multi-user spatial multiplexing. The single-user spatial multiplexing is also referred to as a single user MIMO (SU-MIMO). The multi-user spatial multiplexing is also referred to as a spatial division multiple access (SDMA) or a multi user MIMO (MU-MIMO). A capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel can be decomposed into independent channels. If the number of Tx antennas is Nt and the number of Rx antennas is Nr, then the number of independent channels is Ni where Ni≦min{Nt, Mr}. Each independent channel can be referred to as a spatial layer. A rank represents the number of non-zero eigen-values of the MIMO channel and can be defined as the number of spatial streams that can be multiplexed. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that the data can be transmitted at a high speed without increasing a system bandwidth.
The beamforming is used to add a weight factor to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. The weight factor can be expressed by a weight vector. Two or more weight vectors can be expressed by a weight matrix. The weight vector is referred to as a precoding vector. The weight matrix is referred to as a precoding matrix. Channel dependent precoding is a precoding method using a weight factor depending on the channel condition. The channel dependent precoding uses a weight factor suitable for the channel condition in order to maximize capacity of a transport channel. Channel information for the channel dependent precoding can be obtained using a sounding channel, a codebook, channel quantization, etc. A system using codebook-based precoding among a variety of precoding methods creates a codebook set that can reflect the channel condition, and selects a codebook that maximizes a reception capacity of the transport channel. In general, the reception capacity of the transport channel can increase in proportion to the number of codebooks of the codebook set.
When the OFDM system applies the channel dependent precoding, optimal performance can be obtained using a weight factor for each subcarrier. However, the use of the weight factor for each subcarrier may result in a significantly large overhead caused by signaling. The overhead caused by control signaling when the weight factor for each subband is used by dividing a whole frequency band into a subband which is a group of consecutive subcarriers. A user equipment (UE) reports to a base station (BS) a preferred codebook and a channel condition for each subband, and the BS performs scheduling by considering the reported information. In general, the BS improves system performance by assigning a subband having a good channel condition to the UE. That is, the subband having a good channel condition has a higher possibility of being assigned than a subband having a poor channel condition. An unnecessary overhead may occur when the UE transmits a channel condition and a codebook not only for the subband having a high possibility of being assigned but also for all subbands having a low possibility of being assigned.
Accordingly, there is a need for a method capable of reducing a transfer amount of control information while ensuring scheduling efficiency in a multiple antenna system.